docs: improved SEO of documentation (#3117)

* docs: added description to documentation pages

* docs: added more descriptions

* docs: finished improving meta description

* docs: added searchbox structured data

* docs: added breadcrumbs structured data

* docs: added how to structured data

* docs: improved 404 page

* docs: added how-to frontmatter option

docs/content/advanced/backend/discounts/index.md

@@ -1,3 +1,7 @@
+---
+description: 'Learn about the discount architecture in the Medusa server. Discounts are used to offer promotions to the user for marketing purposes.'
+---
+
 # Discounts Architecture
 
 In this document, you’ll learn about Discounts architecture and how it works.